In order for ethylene copolymers to have improved physical properties and, at the same time, ensure processability, it is advantageous that the ethylene copolymers have a broader molecular weight distribution and a molecular weight distribution showing two or more peaks in the molecular weight distribution curve.
In attempts to make these ethylene copolymers having improved processability and physical properties, U.S. Pat. No. 4,935,474 discloses a process which uses more than two metallocene catalysts having different reaction rates in one reactor. In this process, a polymer having a relatively broad molecular weight distribution or multimodal molecular weight distribution can be produced, but it is difficult to an ethylene copolymer having various density distributions.
U.S. Pat. No. 3,592,880, European Patent Nos. 057420 and 237294 and GB Patent No. 2020672 disclose slurry-slurry multi-stage polymerization processes, GB Patent No. 1505017, European Patent No. 040992 and U.S. Pat. No. 4,420,592 disclose gas phase-gas phase multi-stage polymerization processes, and GB Patent No. 1532231 and U.S. Pat. Nos. 4,368,291, 4309521 and 4368304 disclose slurry-gas phase multi-stage polymerization processes. Furthermore, PCT International Patent Publication No. WO 9212182 discloses that, in the case of a gas-phase process in a slurry-gas phase process, more than two stages are possible. However, a product produced in said PCT patent shows only a bimodal molecular weight distribution through a two-stage process, this being due to properties of the catalyst and the introduction of hydrogen required due to the catalyst properties, and examples of said patent disclose a process of producing an ethylene copolymer having a limited density of more than 0.930 g/cm3. Thus, the process disclosed in said patent has limitations to producing ethylene copolymer resin for use in various applications, including films having high impact strength.
PCT International Patent No. WO 1994/17112 discloses a process of synthesizing an ethylene copolymer having a broad molecular weight distribution using metallocene and Ziegler-Natta catalysts in a solution polymerization process. However, in said patent, because the ethylene copolymer shows only a bimodal molecular weight distribution, there is a limitation in improving the physical properties of the polymers by improving the process.
U.S. Pat. No. 6,277,931 discloses a process of polymerizing ethylene having a bimodal molecular weight distribution using two kinds of catalysts (metallocene and Ziegler-Natta) in a solution polymerization process.
PCT International Patent Publication No. WO 2006/048257 discloses a process for producing an ethylene copolymer having a broad molecular weight distribution and trimodal molecular weight distribution through three reactors. In said PCT patent, the ethylene copolymer having a trimodal broad molecular weight distribution is produced by partially synthesizing a high-molecular-weight, high-density polyethylene in a prepolymer reactor, which is placed before a slurry process in a slurry-gas phase process, and then subjecting the prepolymer to the slurry-gas phase process. However, there is a shortcoming in that, when the high-molecular-weight moiety of the polymer resin has high density, it adversely affects the impact resistance of films in view of the whole moiety of the resin.